• Increasing temperature enhances brittleness, fracture velocity, toughness, load-bearing capacity, roughness, and fracture process zone radius in clay-rich mudstone.
• Pure mode II tensile fractures exhibit the highest velocities, while shear fractures show the lowest velocities and smoothest surfaces.
• Confining pressure of 4 MPa significantly improves shear fracture toughness by up to 119.7% and reduces surface roughness by up to 30.4%.
• Findings provide critical insights for geothermal energy, oil and gas exploration, and underground construction safety.